Prepare and clean the mud crabs by washing it in running water and using a toothbrush, clean the shells specially the abdomen, claws and feet.
Chop and divide each crab into four pieces. Set aside.
Heat sesame oil in a large frying pan over medium heat.
Saute garlic and ginger until fragrant for 1 minute.
Add in oyster sauce, water, salt and sugar.
Bring to a boil then simmer until the sauce is slightly thickened.
Add in the crabs and spring onion and stir cook to coat the crabs with the sauce.
Cover and simmer until the crabs are cooked. Serve hot.